Optimization Of Micro-nano Ozone Disinfection Based On Operation Analysis Of Wastewater Disinfection Processes

Microbial pollution widely existing in various water bodies is a great threat to human health.The public’s attention to the transmission control of pathogenic microorganisms has been pushed to another climax by the outbreak of Corona Virus Disease 2019.The disinfection unit of wastewater treatment plants which become the collection and inactivation node of pathogenic microorganisms,is the key to ensure the safety of reuse water.However,there is a lack of comprehensive summary of regional actual disinfection methods and their operation situation at present.Furthermore,the specific potential problems in the application of each disinfection form are not clear yet.In view of the above-mentioned facts,this paper comprehensively analyzed the actual disinfection process and operation status by focusing on the wastewater treatment plants in Taihu Lake Basin.And two traditional disinfection methods were selected by various comparisons.The disinfection effects of Escherichia colli（E.coli）in simulated and real wastewater were studied for sodium hypochlorite（Na Cl O）solution and O₃micron bubble（O₃-MBs）.On this basis,the optimization was carried out for the application limitations such as insufficient mass transfer efficiency of O₃-MBs.Therefore,O₃ disinfection was coupled with micro nano aeration technology to form O₃ micro nano bubbles（O₃-MNBs）,whose features and performance were investigated in more depth.The main research results of this paper were as follows:（1）The 56 wastewater treatment plants investigated were mainly disinfected by Na Cl O solution,about 90%of whose projects were involved in the method of Na Cl O solution disinfection,supplemented by Cl O₂,UV and O₃.In the 54 projects only disinfected with Na Cl O solution,the average effective chlorine dosage was 3.3 mg/L.The projects with effective chlorine dosage no more than 5 mg/L accounted for 74%.And the projects with disinfection contact time no more than 30 min accounted for 71%.The disinfection contact time was insufficient in most wastewater treatment plants.The average value of residual chlorine in the effluent was 0.7 mg/L.Moreover,the proportion of the projects with residual chlorine concentration above 0.2 mg/L reached 90%,which posed a serious threat to subsequent water bodies.In terms of the practical disinfection effect of the investigated wastewater treatment plants,the effect of O₃ disinfection was the most stable.（2）Laboratory simulation results showed,compared with that of 2.5 mg/L effective chlorine,the inactivation rate of 5.0 mg/L effective chlorine was not significantly accelerated,and it still took nearly 20 minutes to reach 100%inactivation rate,whereas the increment of residual chlorine reached 2.8 mg/L.The existence of humic acid（HA）had a great impact on the effects of both disinfection methods.0.2 mg/L HA reduced the terminal inactivation capacity of O₃-MBs and Na Cl O solution by 5.02 lg and 1.29 lg respectively and the reaction consumption of O₃-MBs by HA was faster than that of Na Cl O solution.However,the form of micron bubbles limited the exertion of O₃ oxidation capacity and led to ineffective loss.20.9mg/L ammonia nitrogen had little effect on disinfection effect and did not accelerate the consumption rate of disinfectant,for which the total chlorine concentration had hardly decreased.In E.coli spiked experiment of real wastewater,0.5 mg/L O₃-MBs reached the same disinfection result as 1.0 mg/L effective chlorine.Yet the residual chlorine concentration of the latter was higher than 0.2 mg/L.The logarithmic model fitting results of E.coli logarithmic removal rate and CT value showed that the CT value required by Na Cl O solution was 1.45times that of O₃-MBs solution to achieve the same effect.The results of flow cytometry showed that the cells were further completely broken after transition to the apoptotic state during O₃-MBs treatment,whose inactivation mechanism was different from that of Na Cl O solution.（3）The form of micro nano bubbles（MNBs）optimized O₃ disinfection mainly by reducing gas loss and enhancing oxidation mass transfer capacity.On the one hand,when the adding concentration of O₃ was 16.11 mg/L in the gas phase,the O₃ escape rate of O₃-MNBs group was 35.19%lower than that of O₃-MBs group in 15 minutes.After increasing O₃ dosing concentration to 60.6 mg/L in the gas phase,the instantaneous escape rate of O₃-MNBs still remained 12.55%lower.Under the air intake of 5 mg/min,micro nano aeration promoted the dissolution of 18.07 mg O₃ at 10 min to reduce losses.On the other hand,the existing form of MNBs also accelerated the decomposition of O₃,which helped to improve the mass transfer efficiency.There were four mechanisms such as MNBs explosion cooperating with O₃decomposition to produce hydroxyl radical in O₃-MNBs disinfection system,in which MNBs had a very long retention time and consequently led to the ability of continuous disinfection.1mg/L O₃-MNBs could remove 3 lg more E.coli than O₃-MBs of the equal concentration within1 min.The form of O₃-MNBs also alleviated the inhibitory effect of HA on O₃ disinfection.And the flow cytometry results showed that the destruction of cell esterase and DNA was aggravated via O₃-MNBs treatment.